(define even
  (fun ([x Int] [-> Bool])
    (if (= x 0)
        true
        (if (= x 1)
            false
            (odd (- x 1))))))

(define odd
  (fun ([x Int] [-> Bool])
    (if (= x 0)
        false
        (if (= x 1)
            true
            (even (- x 1))))))

(print (even 10))                       -- true
(print (even 11))                       -- false
(print (odd 10))                        -- false
(print (odd 11))                        -- true
